A method and apparatus for testing antibiotic susceptibility by measuring the forward light scattering of a series of samples containing standard and test samples of bacteria colonies affected by various antibiotic elutions is described in commonly assigned patent application Ser. No. 281,946, filed Aug. 18, 1972 now U.S. Pat. No. 3,832,532 A photometering analyzer measures the light scattering characteristics of each of a series of samples traversed on a carriage past a photodetecting station. The analyzer prints out a range of readings indicative of the characteristic being measured. The analyzer also provides a reading of a preliminary sample concentration, which is also obtained by light scattering. This analyzer includes various optical and electronic components which would cause erroneous readings if any or some of them should malfunction. An object of this invention is to provide a means for readily determining if such a light scattering photometering analyzer is accurately functioning.